The ability to produce monoclonal antibodies has revolutionized diagnostic and therapeutic medicine. Monoclonal antibodies are typically produced by immortalization of antibody-producing mouse lymphocytes thus ensuring an endless supply of cells which produce mouse antibodies. However, for many human applications, it is desirable to produce human antibodies. For example, it is preferable that antibodies which are administered to humans for either diagnostic or therapeutic purposes are human antibodies since administration of human antibodies to a human circumvents potential immune reactions to the administered antibody, which reactions may negate the purpose for which the antibody was administered.
In addition, there exists certain situations where, for diagnostic purposes, it is essential that human antibodies be used because other animals are unable to make antibodies against the antigen to be detected in the diagnostic method. For example, in order to determine the Rh phenotype of human red blood cells (RBCs), human sera that contains anti-Rh antibody must be used since other animals cannot make an antibody capable of detecting the human Rh antigen.
The production of human antibodies in vitro by immortalizing human B lymphocytes using Epstein Barr virus (EBV)-mediated transformation or cell fusion has been fraught with technical difficulties due to the relatively low efficiency of both EBV-induced transformation and cell fusion when compared with the murine system. To overcome these problems, processes have been developed for the production of human antibodies using M13 bacteriophage display (Burton et al., 1994, Adv. Immunol. 57:191-280). Essentially, a cDNA library is generated from mRNA obtained from a population of antibody-producing cells. The mRNA encodes rearranged immunoglobulin (Ig) genes and thus, the cDNA encodes the same. Amplified cDNA is cloned into M13 expression vectors creating a library of phage which express human Fab fragments on their surface. Phage which display the antibody of interest are selected by antigen binding and are propagated in bacteria to produce soluble human Fab Ig. Thus, in contrast to conventional monoclonal antibody synthesis, this procedure immortalizes DNA encoding human Ig rather than cells which express human Ig.
There are several difficulties associated with the generation of antibodies using bacteriophage. For example, many proteins cannot be purified in a non-denatured state, in that purification procedures necessarily involve solubilization of protein which may render some proteins permanently denatured with concomitant destruction of antigenic sites present thereon. Such proteins thus cannot be bound to a solid phase and therefore cannot be used to pan for phage bearing antibodies which bind to them. An example of such a protein is the human Rh antigen.
To solve the problem, a method was developed wherein intact RBCs were used as the panning antigen (Siègel et al., 1994, Blood 83:2334-2344). However, it was discovered that since phage are inherently “sticky” and RBCs express a multitude of antigens on the cell surface, a sufficient amount of phage which do not express the appropriate antibody on the surface also adhere to the RBCs, thus rendering the method impractical for isolation of phage which express antibody of desired specificity.
De Kruif et al. (1995, Proc. Natl. Acad. Sci. USA 92:3938-3942) disclose a method of isolating phage encoding antibodies, wherein antibody-expressing phage are incubated with a mixture of antigen-expressing cells and cells which do not express antigen. The antibody-expressing phage bind to the antigen-expressing cells. Following binding with phage, a fluorescently labeled antibody is added specifically to the antigen-expressing cells, which cells are removed from the mixture having antibody-expressing phage bound thereto. The isolation of fluorescently labeled cells is accomplished using the technique of fluorescently-activated cell sorting (FACS), an expensive and time-consuming procedure.
There remains a need for a method of isolating recombinant proteins, preferably antibodies, which is rapid and economical, and which will provide a vast array of protein-binding proteins useful for diagnostic and therapeutic applications in humans.